The present invention relates to power converters and more particularly to a control system for use with a voltage rectifier for minimizing voltage on a DC bus without distorting utility grid AC voltages and currents.
To illustrate the effects of distorting currents on a utility power grid, consider FIG. 11 wherein a utility source 10 is shown connected at a point of common coupling (PCC) (i.e., a utility-customer connection point) to a load 12 (e.g., a first utility customer) and other loads (e.g., other utility customers) represented collectively by numeral 14. The utility source 10 includes a finite internal impedance L.sub.s. Due to the internal impedance L.sub.s, when load 12 draws a non-sinusoidal current from the source 10, the waveform at the PCC becomes distorted with harmonic currents which can cause machinery and equipment connected at the other loads 14 to malfunction.
In addition to voltage waveform distortion at the PCC, other problems related to harmonic currents include additional heating and possibly over voltages in utility distribution and transmission equipment, errors in metering and malfunctioning of utility relays, interference with communication and control signals and equipment damage from voltage spikes created by high frequency resonances.
Unfortunately, harmonic or non-linear loads comprise an ever increasing portion of the total load for a typical industrial plant. In fact, by 1992, harmonic loads had become such a pervasive problem that the Institute of Electrical and Electronic Engineers (IEEE) recommended stringent harmonics standards, including strict utilities limitations, in a document referred to in the industry as IEEE Standard 519 which has generally been accepted in North America. Standard 519 was written with the general understanding that harmonics should be within a reasonable limit at the PCC and therefore puts limits on individual and total (i.e., distortion from all loads connected at a PCC) harmonic distortion.
One potential source of utility grid distortion includes power electronics required to modify utility voltages for driving motors. Generally, power electronic systems for receiving and converting utility voltages into AC voltages suitable for driving an AC motor include two converter stages, the first converter stage being a rectifier stage and the second converter stage being an inverter stage. The rectifier stage receives and converts the AC utility voltages to DC voltage and provides the DC voltage across positive and negative DC buses. The inverter stage receives and converts the DC voltage to AC voltages, usually at a different frequency and amplitude than the utility voltages, and provides the converted AC voltages to motor terminals to drive a motor.
To convert the AC utility voltage to DC voltage, a common rectifier configuration includes at least six diodes arranged to form three parallel rectifier legs between the positive and negative DC buses, each leg including a pair of series connected diodes. Each utility AC input line is connected to one of the legs between an associated diode pair. A charging bus capacitor is linked between the positive and negative DC buses. The diodes cooperate to pass each positive half cycle of AC voltage to the DC bus and to invert and pass each negative half cycle of AC voltage to the DC bus. The result is that an essentially DC current is provided on the DC bus and the bus capacitor charges to a DC voltage V.sub.dc.
To convert the DC voltage V.sub.dc to AC voltage and control both frequency and amplitude, the inverter stage typically includes at least six switching devices (e.g. IGBT, BJT, etc.) arranged to form parallel legs between the positive and negative DC buses, each leg including a pair of series connected devices. A node between each pair of inverter switches is linked to a separate one of the motor stator windings at a motor terminal.
The inverter devices in each leg are alternately turned on and off such that a series of high frequency voltage pulses are provided at an associated terminal. The devices are turned on and off such that the fundamental value of the resulting high frequency pulses is a low frequency alternating voltage at the terminal. To generate device trigger signals for turning the devices on and off three modulating signals, a separate modulating signal corresponding to each of the inverter legs, are provided for comparison to a high frequency triangle carrier signal. When a modulating signal is greater than the carrier signal, a trigger signal causes an associated lower device to turn off and an associated upper device to turn on thereby connecting an associated supply line to the positive DC bus. When a modulating signal is less than the carrier signal, the trigger signal causes an associated upper device to turn off and an associated lower device to turn on thereby connecting an associated supply line to the negative DC bus.
A modulation index Mi is the ratio of the peak value of a modulating signal and a peak value of the triangle carrier signal. Typically the low frequency alternating voltage changes linearly with the modulating signal up to a value of one for M.sub.i. As index Mi is increased above one, while the alternating voltage will still increase, it increases at a fraction of the index M.sub.i rate. Therefore, where index Mi is between zero and one, the inverter is said to be operating in a linear region of operation.
The low frequency alternating voltages at the terminals cooperate to drive the motor as is well known in the art. When an inverter is used to drive a motor the inverter is said to be operating in a motoring mode.
In addition to driving the motor in a motoring mode, an inverter can also be used in a "braking" mode to reduce motor speed. During braking, the terminal voltages are provided at a speed which is less than a motor rotor's mechanical speed. In this case, instead of providing current to the motor stator windings, the motor operates as a generator providing current back through the inverter to the DC bus. In essence, the inverter acts as a rectifier during braking tending, like the rectifier connected to the utility grid, to charge the bus capacitor.
With a diode bridge the current direction from the grid to the bus capacitor cannot be reversed. Therefore, some mechanism must be provided to dissipate energy returned to the DC bus during braking, otherwise the voltage across the bus capacitor can reach destructive levels. One common way to dissipate braking current is to provide a switch in series with a braking resistor in parallel with the bus capacitor. If capacitor charge exceeds a threshold voltage level, the switch is closed so that the braking resistor dissipates braking energy.
As well known in the art, during steady state motoring at light load the DC voltage V.sub.dc using a diode bridge rectifier is approximately .sqroot.2 times the line-to-line utility voltage VII. For example, if the line-to-line utility voltage VII is 460 volts, voltage Vdc is approximately 650 volts. If VII increases to 504 volts (i.e. a 10% increase), Vdc increases to 715 volts. During braking the DC voltage Vdc reaches the threshold voltage at which a resistor brake is turned on. In many rectifier configurations, the threshold voltage at which the resistor is switched in parallel with the charging capacitor to discharge braking energy is approximately 750 volts. Thus, with a diode bridge rectifier, the DC bus voltage often fluctuates during operation (e.g. between 650 and 750 volts).
While diode bridge rectifiers are relatively simple, inexpensive, easy to operate and do not cause appreciable utility grid distortion, they have at least two important shortcomings. First, because DC voltage Vdc changes as a function of line-to-line utility voltage VII, operating characteristics of other equipment linked to the utility can be disadvantageously affected by utility voltage fluctuations. For example, if voltage VII is initially at 480 volts so that the DC voltage Vdc is 678 volts, the motor terminal high frequency voltage pulses alternate between +339 and -339 volts and a first maximum speed is achievable. However, if voltage VII is reduced from 480 volts to 460 volts such that the DC voltage Vdc is reduced from 678 volts to 650 volts, the high frequency voltage pulses at the motor terminals fluctuate between +325 and -325 volts and a second maximum speed which is less than the first maximum speed is achievable.
Second, because a braking resistor is used to dissipate braking current, diode bridge rectifiers waste energy and are relatively inefficient. This is particularly true in applications where braking is performed frequently.
One way to overcome the shortcomings associated with a diode bridge rectifier has been to construct a controllable regenerative rectifier, also referred to herein and within the industry as a switch-mode rectifier. Like a typical inverter, a regenerative rectifier includes at least six switching devices arranged to form three parallel legs between the positive and negative DC buses, each leg including two series connected switching devices. Six separate diodes are arranged in inverse parallel relationship with the switching devices, a separate diode connected to each switching device. A DC bus capacitor is positioned between the positive and negative DC buses. A separate one of the three utility AC lines is connected to a node between an associated pair of series devices via an input reactor or inductor. For simplicity, utility line voltages will be referred to hereinafter as utility AC voltages and rectifier input voltages will be referred to as rectifier AC voltages.
In operation, initially the switching devices are turned off and the inverse parallel diodes operate as a diode bridge rectifier to charge the DC bus capacitor during an initial charging period. After the initial charging period and during a motoring operation, the switching devices are controlled like an inverter to generate rectifier AC voltages on the rectifier inputs which lag the utility AC voltages and have a slightly smaller amplitude. Where the rectifier AC voltages are in phase with and slightly less than the utility AC voltages, currents pass through the input reactors from the AC to the DC sides of the rectifier thereby providing currents to charge the bus capacitor and drive the motor.
To generate the rectifier AC voltages the rectifier is controlled in a manner similar to control of the inverter. To this end, rectifier devices in each leg are alternately turned on and off such that a series of high frequency voltage pulses are generated at an associated rectifier input. The fundamental value of the resulting high frequency voltage pulses is a low frequency alternating voltage at the rectifier input. By controlling the high frequency pulses, the desired low frequency fundamental alternating voltage can be generated.
In addition to being used during motoring, regenerative switch mode rectifiers can be used during braking to return energy recovered from motor inertia to the utility grid, hence the term "regenerative". During regeneration, as during motoring, rectifier switches are turned on and off to generate high frequency voltage pulses on the utility lines, the fundamental values of which result in low frequency alternating voltages on the utility lines which are preferably in phase with the rectifier AC voltages. However, instead of having amplitudes which are slightly less than the amplitudes of the utility AC voltages and phases which lag the utility AC voltages, the low frequency alternating voltages during regeneration have amplitudes which are slightly larger than the amplitudes of the utility AC voltages and have phases which lead the utility AC voltages so that current passes from the DC to the AC sides of the rectifier. In this manner, braking power is returned to the utility grid.
Drawing current from and returning current to the utility grid can distort utility AC voltages and currents if the DC bus voltage is insufficient to generate low frequency alternating voltages at the rectifier inputs which are in phase and slightly greater than the utility supplied rectifier input voltages during regeneration and slightly less than the utility supplied rectifier input voltages during motoring.
To ensure required rectifier input voltages and minimize utility harmonics, DC bus voltage Vdc must be of a sufficiently large magnitude that voltage Vdc covers the line-to-line utility AC voltage VII plus any voltage drop across rectifier input reactors. To this end DC voltage Vdc is often chosen to be: ##EQU1##
The factor 1.15 accounts for possible voltage drop across rectifier input reactors and for utility line voltage fluctuations (e.g. .+-.10%). In this case a utility AC voltage of 460 volts requires voltage Vdc of at least 748 volts.
Regenerative controllable rectifiers can be controlled to maintain a desired DC bus voltage independent of fluctuations in the utility AC voltage. To this end, a regenerative rectifier can be controlled as a function of utility AC voltages to alter the widths of the high frequency voltage pulses provided at the rectifier inputs. For example, assuming line-to-line utility voltage VII of 460 volts which is controlled to provide 748 volts Vdc, if the line-to-line utility voltage increases to 480 volts, DC voltage Vdc can be maintained at 748 volts by modifying high frequency pulse widths so that, despite change in utility line voltages, the currents drawn from the utility lines remain the same. Similarly, if regeneration provides charging current to the bus capacitor tending to increase Vdc, the regenerative rectifier can be controlled to modify the high frequency pulse widths to a suitable degree to maintain an essentially constant DC bus voltage Vdc.
Thus regenerative rectifiers are both more efficient than non-regenerative rectifiers and can be controlled to provide a constant DC bus voltage which does not fluctuate with utility voltage fluctuations and motor braking.
Unfortunately, regenerative rectifiers also have at least one important shortcoming. During inverter switching to convert DC bus voltage Vdc to AC voltage for driving the motor, switching losses are directly related to DC bus voltage level. Switching losses increase and less efficient switching occurs as DC bus voltage Vdc increases. Thus, higher DC bus voltages Vdc associated with regenerative rectifiers result in greater switching losses than non-regenerative rectifiers. In addition, increased DC bus voltage Vdc also increases overvoltages on the motor due to reflections on long supply lines.